The present invention relates to binoculars and in particular to binoculars having a focus that is fixed at infinity.
Conventional binoculars are constructed so as to allow a variation in distance between the eyepiece lens and associated objective lens of each lens set so that the focus of each lens set can be modified in accordance with the distance of a user from the object being viewed. Normally there is a mechanism for simultaneously adjusting the focus of each lens set. The structure required to allow this adjustability of the focus is usually expensive and adds substantially to the cost of the binoculars. For binoculars used by many different people, this also results in constant readjustment of the focus.
Both medical doctors and scientists recognize that eyes have a natural ability to make some adjustment to distance when focus is provided for an infinite distance. A normal healthy person whose eyes have not been damaged by age or disease can adjust approximately four diopters. It has been further determined that the range of ability to adjust, when focusing at an infinite distance, has nothing to do with the objective aperture of each set of lenses of the binoculars but rather is determined by the magnification developed by each set of lenses. In particular, the minimum distance to which a normal person can adjust is directly proportional to the square of the magnification divided by the diopter adjustment that a particular person can make. Therefore, where the magnification is 7 and a person can adjust four diopters, such a person can actually adjust an image focused at infinity such that they can view an image at 12.25 meters with apparent focus. As the magnification increases the ability of the eye to adjust to an image at infinite focus increases to larger distances. For example, at a magnification of 10 the minimum adjustable distance is approximately 25 meters.
Since most viewers of binoculars are utilizing low magnification and tend to view objects mostly at distances greater than this minimum adjustable distance, applicant has determined that the lens sets can be fixed and positioned to provide an infinite focus and yet allow a substantial variation in viewing due to the natural adjustability of the eye. Such a fixed focus allows a substantial savings in the cost of production of such binoculars and eliminates the need to constantly readjust the binoculars, especially when used by multiple viewers.